U.S. patent number 7,165,849 [Application Number 10/979,199] was granted by the patent office on 2007-01-23 for projector with auto focus device.
This patent grant is currently assigned to Fujinon Corporation. Invention is credited to Takao Araki, Tomonari Masuzawa, Yasuhiro Miwa, Tatsuo Saito.
United States Patent |
7,165,849 |
Masuzawa , et al. |
January 23, 2007 |
Projector with auto focus device
Abstract
An image displayed on a liquid crystal panel is projected on a
projection surface by a projection lens such that the optical axis
of the projection lens is perpendicular to the projection surface.
An optical axis of a distance measurement light and an optical axis
of a light receiving lens are each inclined a specified angle with
respect to the optical axis of the projection lens to be not
perpendicular to the projection surface so as to reduce noise
caused by specularly reflected light that otherwise would be
detected by the distance measurement unit when a highly reflective
whiteboard is utilized as the projection surface. The distance
measurement unit is formed by unitizing a light emitting element, a
light emitting lens, the light receiving lens, and a light
receiving element, and the distance measurement unit is held with
the projection lens by a common holder.
Inventors: |
Masuzawa; Tomonari (Saitama,
JP), Miwa; Yasuhiro (Saitama, JP), Saito;
Tatsuo (Saitama, JP), Araki; Takao (Saitama,
JP) |
Assignee: |
Fujinon Corporation (Saitama,
JP)
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Family
ID: |
34544394 |
Appl.
No.: |
10/979,199 |
Filed: |
November 3, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050099609 A1 |
May 12, 2005 |
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Foreign Application Priority Data
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Nov 6, 2003 [JP] |
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2003-377430 |
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Current U.S.
Class: |
353/101;
348/E5.137; 353/69 |
Current CPC
Class: |
G01C
3/10 (20130101); G03B 21/53 (20130101); H04N
5/74 (20130101); H04N 9/317 (20130101); H04N
9/3185 (20130101) |
Current International
Class: |
G03B
21/00 (20060101); G03B 3/00 (20060101) |
Field of
Search: |
;353/69,70,100,101 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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5-346569 |
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Dec 1993 |
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JP |
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2003-161869 |
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Jun 2003 |
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JP |
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Primary Examiner: Fuller; Rodney
Attorney, Agent or Firm: Arnold International Arnold; Bruce
Y.
Claims
What is claimed is:
1. A projector for projecting an image on a projection surface
through a projection lens, including an active type distance
measurement device which obtains distance measurement data
corresponding to a distance between said projection lens and said
projection surface, said distance measurement device comprising: a
light emitting element for emitting a distance measurement light; a
light emitting lens for directing said distance measurement light
from said light emitting element to said projection surface; a
light receiving lens for transmitting said distance measurement
light reflected from said projection surface; and a light receiving
element for receiving said distance measurement light passed
through said light receiving lens, wherein an optical axis of said
distance measurement light from said light emitting lens to said
projection surface and an optical axis of said light receiving lens
are parallel to each other and inclined a specified angle with
respect to an optical axis of said projection lens such that light
that is diffused in transiting the light emitting lens and
specularly reflected by the projection surface does not enter said
light receiving element while entering said light receiving
lens.
2. A projector according to claim 1, further including an image
displaying panel for displaying said original image, which is
disposed between said projection lens and a projecting light
source.
3. A projector according to claim 2, wherein said light emitting
element, said light emitting lens, said light receiving lens, and
said light receiving element are inclined with respect to an
optical axis of said projection lens.
4. A projector according to claim 2, wherein said optical axis of
said projection lens is perpendicular to said projection surface,
the center of said image displaying panel is shifted away from said
optical axis of said projection lens, and said optical axis of said
distance measurement light from said light emitting lens to said
projection surface and said optical axis of said light receiving
lens are inclined to a direction toward a center of said projected
image.
5. A projector according to claim 2, wherein said light emitting
element is an infrared emitting diode, and said light receiving
element is a position sensitive detector.
6. A projector according to claim 2, wherein said distance
measurement device is assembled as a unit and is held with said
projection lens by a common holder.
7. A projector according to claim 6, wherein said holder holds a
barrel which contains said projection lens, and a support plate to
which said distance measurement device are fixed, said barrel and
said support plate being parallel to each other, and said distance
measurement device being inclined with respect to said support
plate.
8. A projector according to claim 7, further including a focus
motor for focusing said projection lens and a zoom motor for
zooming said projection lens, said focus motor and said zoom motor
being fixed to said support plate.
9. A projector according to claim 2, wherein said optical axis of
said distance measurement light from said light emitting lens to
said projection surface and said optical axis of said light
receiving lens are inclined with respect to the horizontal
direction.
10. A projector according to claim 9, wherein said light emitting
lens and said light receiving lens are arranged vertically.
11. A projector according to claim 9, wherein said light emitting
lens and said light receiving lens are arranged horizontally.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projector having an auto focus
device.
2. Description Related to the Prior Art
Many kinds of projectors for projecting images to a screen such as
a slide projector and a liquid-crystal projector are utilized. In
back projection type projectors which incorporates the screen,
there is no need to perform focusing of a projection lens for every
use because a distance between the projection lens and the screen
stays constant. However, in general projectors which projects
images from front side of the screen, there is need to perform
focusing of a projection lens according to the distance between the
projection lens and the screen.
The projectors which incorporate an auto focus device to enable the
automatic focusing of the projection lens are disclosed in
Laid-Open Japanese Patent Applications 5-346569 and 2003-161869. As
the auto focus device of the projector, it is often used that an
active type distance measurement device which measures the distance
between the projection lens and the screen by triangulation such
that the device projects a distance measurement light to the screen
which is the projection surface for the image and receives the
diffuse reflected light from the screen.
As shown in FIG. 6, in general the projector 2 is mounted on a
table 3, and the screen 4 is often set at high place so that the
projected image can be seen easily. In this case, if the whole
projector 2 is inclined upward for adjusting the center of the
projected image and center of the screen 4, the projected image
becomes a trapezoid shape because the image is expanded in the
upper part of the screen. To avoid this problem, in the figure, the
projection lens 5 is shifted upward with respect to center of a
liquid crystal panel 6 which displays images for projecting with
that the optical axis 5a of the projection lens 5 is horizontal
(perpendicular to the screen 4). Accordingly, the strain-free image
can be projected on the screen 4, in spite of that an optical axis
5b of center of the image, which connects the center of the liquid
crystal panel 6 and the center of the screen 4, is inclined upward
with respect to an optical axis 5a of the projection lens 5.
In the active type distance measurement device used in Laid-Open
Japanese Patent Applications 5-346569 and 2003-161869, as described
in FIG. 7, near-infrared light from an infrared emitting diode
(IRED) 10 is projected to the projection surface 9 through a light
emitting lens 11. In case that the screen is used as the projection
surface 9, an spot image (spot area S1) is formed on the surface of
the screen by irradiation of the distance measurement light,
because the surface is fine coarse and has diffusion reflection
property. A light receiving lens 12 is provided at a position
distant from center of the light emitting lens by a base length L.
The diffuse reflected light from the spot area S1 enters to the
light receiving lens 12, and an spot image is focused on a
photoelectric surface of a light receiving element 13 provided back
of the light receiving lens 12. Note that it only needs that the
spot image on the screen is in a field angle of the light receiving
lens 12. Therefore, there is no need that an optical axis 12a of
the light receiving lens 12 is parallel to an optical axis 11a of
the light emitting lens 11, even though there often be a case that
an optical axis 12a of the light receiving lens 12 is parallel to
an optical axis 11a of the light emitting lens 11, as described in
the figure.
In general, a PSD (Position Sensitive Detector) is used as the
light receiving element 13, which outputs a pair of electrical
signals corresponding to a position (center of gravity) of the spot
image imaged by the light receiving lens 12. The PSD has a function
to discriminate the position of incidence of light in direction of
the base length L, and the image forming position of the spot image
on the photoelectric surface corresponds to the distance between
the projection lens and the screen by triangulation. Therefore, a
distance measurement signal corresponding to the distance between
the projection lens and the screen can be generated based on the
pair of electrical signals from the PSD, with being immune to
amount of light entered into the PSD. The automatic focusing is
operated such that a focus motor is driven according to the
distance measurement signal to move a focus lens of the projection
lens system in direction of the optical axis 5a.
Recently, whiteboards are often used as a substitute for the screen
when the projector is used in offices. The whiteboard has a surface
which is smoother and has higher reflectivity than the normal
screen having diffusion reflection property, to enable writing down
by markers and erasing by erasers. Accordingly, the distance
measurement light emitted from the distance measurement device is
reflected at a considerably high intensity at the whiteboard and
enters into the light receiving element 13. In this time, the
distance measurement light from the light emitting lens 11 to the
projection surface 9 includes not only the effective distance
measurement light formed as a beam. Because the light emitting lens
11 is not absolutely transparent to near-infrared light, the light
emitting lens 11 emits diffused light with the effective distance
measurement light. In addition, diffuse reflected light is often
emitted from for example components mounted back of the light
emitting lens 11.
As shown in FIG. 7, most of these noise lights are emitted with
gradation of intensity as high as closer to a center of a
surrounding area S2 which surrounds the spot area S1 to which the
effective distance measurement light is emitted, and as low as
closer to a periphery of the surrounding area S2. Ordinarily, the
intensity of the noise light is very lower than that of the
effective distance measurement light, therefore the noise light
from the surrounding area S2 hardly reaches to the light receiving
lens 12 when the normal screen is used as the projection surface 9.
However, when the whiteboard is used, part of the noise light which
is emitted to the surrounding area S2 becomes to have many
components which are regularly reflected at the surface of the
whiteboard (the light which has an exit angle being equal to an
incidence angle). As shown in figure as "regularly reflected noise
light", part of diffused light reaches to the light receiving
element 13 by entering into the light receiving lens 12 at an angle
different from that of the effective distance measurement light
with being little attenuated. As described above, the PSD which is
often used as the light receiving element 13, outputs electrical
signals corresponds to the incidence position of the distance
measurement light with being immune to intensity of the distance
measurement light, therefore the noise light becomes a major factor
of erroneous distance measurement.
Even if the case that other photoelectric sensors are used as the
light receiving element 13, it is also difficult to distinguish the
distance measurement light and the noise light according to the
intensity of the distance measurement light, because the normal
screen or the whiteboard is used as the projection surface 9
according to situations, and the distance from the projector is
different in different cases. In addition, measures for erroneous
distance measurement from the noise light rises cost of the
projector.
As an example of the distance measurement is executed with the
condition that the distance between the projector and the
whiteboard is 1 meter, and the light emitting optical axis 11a and
the optical axis 12a of the light receiving lens 12 are parallel to
the optical axis 5a of the projection lens 5, and with inclining
the projector upward and downward. In FIG. 8, a horizontal axis
represents projective angle of the projection lens to the
whiteboard, in which the condition that the optical axis 5a of the
projection lens 5 and the optical axis 11a of the light emitting
lens 11 is perpendicular to the whiteboard is shown at [0.degree.],
the condition that the optical axis 11a is inclined mostly upward
with respect to the whiteboard is shown at [-10.degree.], and the
condition that the optical axis 11a is inclined mostly downward
with respect to the whiteboard is shown at [10.degree.]. A vertical
axis represents digital value of distance signal (larger value
corresponds to shorter distance) calculated based on the signal
from the light receiving element (PSD) 13, each value corresponds
to the distance to the whiteboard one to one.
Because the distance between the whiteboard and the projector is
constant at 1 meter, the distance signal is expected to be constant
value (.apprxeq.4875) even if the projection angle is changed.
However, when the projection angle is between [-2.degree.] and
[3.degree.], the distance signal abnormally depends on the
projection angle, even though the mostly constant distance signal
is obtained when the projection angle is more than 3.degree. in
upward or downward direction. The fact means that when the light
emitting optical axis 11a is inclined at [-2.degree. to 3.degree.]
with respect to the whiteboard, the possibility of occurring the
erroneous distance measurement becomes high.
SUMMARY OF THE INVENTION
A main object of the present invention is to provide a projector
whose distance measurement device for determining focusing point of
a projection lens does not perform erroneous distance measurement
even when a surface like a whiteboard which has high surface
reflectance is used as a screen.
Another object of the present invention is to reduce a cost of
manufacturing a projector by making setting up of the projector
more efficient.
In order to achieve the object and the other object, the projector
of the present invention comprises an active type distance
measurement device which obtains distance measurement data
corresponding to a distance between a projection lens and a screen
such that a light emitting element emits a distance measurement
light through a light emitting lens to a projection surface on
which projected image is displayed, and a light receiving element
receives reflected light from the projection surface through a
light receiving lens. In the distance measurement device, an
optical axis of the distance measurement light and an optical axis
of the light receiving lens are inclined to same direction with
respect to an optical axis of the projection lens. It is preferable
that the optical axis of the distance measurement light and the
optical axis of the light receiving lens are inclined to a
direction toward a center of the projected image.
It is also preferable that the distance measurement device is
assembled as a unit and is held with the projection lens by a
common holder.
The projector of the present invention can perform focusing of the
projection lens with accuracy by accurate distance measurement with
the active type distance measurement device even if whiteboards
which has high reflectivity is used as the projection surface,
because noise light does not affect the distance measurement.
In addition, it is easy to maintain relationship between the light
emitting optical axis of the distance measurement light and the
optical axis of the light receiving lens with respect to the
optical axis of the projection lens, because the distance
measurement device is assembled as the unit and is held with the
projection lens by the common holder. This is effective to decrease
the cost of manufacturing the projector.
BRIEF DESCRIPTION OF THE DRAWINGS
The above objects and advantages of the present invention will
become easily understood by one of ordinary skill in the art when
the following detailed description would be read in connection with
the accompanying drawings.
FIG. 1 is a perspective view of a projector of the present
invention;
FIG. 2 is a conceptual diagram illustrating one embodiment of usage
of the projector of the present invention;
FIG. 3 is a perspective view of a projection lens unit of the
projector;
FIG. 4 is a block diagram illustrating electrical composition of an
auto focus device;
FIG. 5 is an explanatory diagram illustrating how to calculate an
inclination angle .theta. of a light emitting optical axis;
FIG. 6 is a conceptual diagram illustrating one embodiment of usage
of a traditional projector;
FIG. 7 is an explanatory diagram illustrating a traditional
distance measurement device;
FIG. 8 is a graph illustrating variation of distance measurement
signal from the distance measurement device when the traditional
projector is inclined;
FIG. 9 is a perspective view of another projector of the present
invention; and
FIG. 10 is a conceptual diagram illustrating one embodiment of
usage of the projector of FIG. 9.
PREFERRED EMBODIMENTS OF THE INVENTION
In a liquid crystal projector 20 shown in FIG. 1, projection lens
21 is exposed from an opening formed on front of a case when a lens
cover is opened. Zooming and focusing the projection lens 21 can be
operated manually by a zooming dial 22 and a focusing dial 23 which
are provided on the case. A mark for automatic focusing is provided
on the focusing dial 23. When the mark is faced with an index on
the case, the projection lens 21 is focused automatically.
The case contains an active type distance measurement device and
lens moving device which moves focus lens of the projection lens 21
based on distance measurement data obtained from the distance
measurement device, so that the projection lens 21 can be focused
automatically. Beside the projection lens 21, there are light
emitting lens 24 and light receiving lens 25 of the distance
measurement device, which are arranged vertically. Distance
measurement light (near-infrared light) is emitted through the
light emitting lens 24 to projection surface of the image along a
fixed optical path, to form a spot image of the distance
measurement light on the projection surface. The spot image is a
distance measurement point. The distance measurement light from the
spot image is received by a light receiving element described
later, through the light receiving lens 25. Note that front of the
light emitting and receiving lenses 24, 25 may be covered by
filters having transparency to near-infrared light.
When the projector 20 is placed horizontally on a mount surface
such as a table, an optical axis 21a of the projection lens 21 is
horizontal to the mount surface, and vertical to a projection
surface 9 such as a screen as shown in FIG. 2. Center of the
projection lens 21 is shifted above center of a liquid crystal
panel 27 as an image displaying panel which displays images for
projecting. Accordingly, the image displayed on the liquid crystal
panel 27 is projected upwardly on the projection surface 9 when a
projecting light source 28 is turned on, and the projected image on
the projection surface 9 is strain-free. Note that a shift
mechanism may be provided to control shift amount of the projection
lens 21 according to vertical interval between the projection
surface 9 and the projector 20.
A light emitting optical axis 24a of the distance measurement
device and an optical axis 25a of the light receiving lens 25 are
inclined upward with respect to the horizontal optical axis 21a of
the projection lens 21. The light emitting optical axis 24a and the
light axis 25a of the light receiving lens 25 can be inclined to
any direction, however, it is practical to incline these optical
axes upward as described above, so that a position where the
distance measurement light is projected is not far away from the
projected image. As described later in detail, the inclination
angle .theta. is determined for preventing that noise light such as
diffuse transmitted light from the light emitting lens 24 itself
and diffuse reflected light from various components mounted back of
the light emitting lens 24, which exist with the effective distance
measurement light emitted from the light emitting lens 24 as a beam
along the light emitting optical axis 24a, is emitted to the
projection surface 9, and direct reflected light and diffuse
reflected light from the projection surface 9 enter to the light
receiving element 13 through the light receiving lens 25.
As shown in FIG. 3, the projection lens 21 is constituted by zoom
lens, which is contained in a barrel 30. A focus ring 31 and a zoom
ring 32 are provided in the barrel 30. Focusing and zooming are
performed by rotating these rings around the optical axis 21a. The
barrel 30 is held by a holder plate 35 which is fixed to a base
plate united with the case of the projector 20.
A lens driving unit 38 is fixed to the holder plate 35 by using
bosses 36 for fixture. The lens driving unit 38 includes a distance
measurement unit 39 which exposes the light emitting lens 24 and
the light receiving lens 25 at front surface and installs IRED and
PSD behind the light emitting lens 24 and the light receiving lens
25, a support plate 43 which holds the distance measurement unit 39
and mounts a focus motor 41 and a zoom motor 42 above and below the
distance measurement unit 39, a focusing gear box 45 in which a
drive gear and reduction gear system engaged with the focus ring 31
is installed, a zooming gear box 46 in which a drive gear and
reduction gear system engaged with the zoom ring 32 is installed,
and a encoder unit 48 which contains rotary encoders for detecting
amounts of rotation of the focus motor 41 and the zoom motor 42. A
circuit board 49 is attached to a side of the lens driving unit 38
by screws.
When setting up the lens driving unit 38, fixation between the
distance measurement unit 39 and the support plate 43 is adjusted
such that all of the light emitting lens 24, the IRED, the light
receiving lens 25, and the PSD are inclined upward such that the
light emitting optical axis 24a and the optical axis 25a of the
light receiving lens 25 are inclined upward at the above stated
inclination angle .theta.. Therefore, the light emitting optical
axis 24a and the optical axis 25a of the light receiving lens 25
become inclined upward at the inclination angle .theta. with
respect to the optical axis 21a of the projection lens 21 in spite
of that the barrel 30 and the lens driving unit 38 are attached to
the holder plate 35 at regular position. In addition, the drive
gears can be accurately engaged with the focus ring 31 and the zoom
ring 32 because the support plate 43 is kept in the regular
position. Accordingly the setting up the lens driving unit 38
becomes efficient.
As shown in FIG. 4, a CPU 52 reads in initial setting data and
adjusting data written in an EEPROM 53, sends commands to a
distance measurement IC 54, a focus lens drive circuit 55, and a
driver 56, to control distance measurement process and focusing
process entirely.
When the distance measurement process is started, the driver 56
send a light emitting command to the IRED 10. According to the
light emitting command, the IRED 10 emits the distance measurement
light. The distance measurement light reflected from the projection
surface 9 enters to the light receiving element 13. An incidence
position on the light receiving element 13 exclusively corresponds
to a distance between the projector 20 to the projection surface 9.
The PSD is used as the light receiving element 13, and outputs a
pair of electrical signals to first and second signal processing
circuits 57, 58. An arithmetic circuit 60 samples output signals
from the first and second signal processing circuits 57, 58 in
synchronization with the light emitting command from the driver 56,
and calculates a distance measurement signal based on output ratio
of the output signals. As described above, by using the output
ratio of the pair of signals, stationary light components entering
to the light receiving element 13 and intensity component of the
distance measurement light are canceled each other. Accordingly,
the distance measurement signal corresponds to the incidence
position of the distance measurement light on the light receiving
element 13 can be obtained.
An output circuit 61 accumulates the distance measurement signal
outputted from the arithmetic circuit 60 in an integrating
capacitor 62. For example when the IRED 10 emits the light hundred
times according to the command from the driver 56, and the
arithmetic circuit 60 samples hundred distance measurement signals
in synchronization with that the IRED 10 emits the light, hundred
distance measurement signals are accumulated in the integrating
capacitor 62. The accumulated distance measurement signals are read
into the CPU 52 via an A/D port. The CPU 52 obtains a distance
signal which is not affected by noises, by calculating the average
of the accumulated distance measurement signals. In addition, it
can obtain more accurate distance signal by executing processes
that these described distance measurement steps is operated as one
distance measurement routine, the distance measurement routine is
operated for example three times, and the three distance signals
obtained by the each distance measurement routine are averaged, as
one distance measurement process.
The CPU 52 outputs focus signal corresponding to the distance
signal obtained by one distance measurement process to the focus
lens drive circuit 55. The focus lens drive circuit 55 drives the
focus motor 41 according to the inputted focus signal, to move the
focus lens 66 which is part of the projection lens 21 from a home
position. The driving of the focus motor 41 is monitored by the
rotary encoder 67 contained in the encoder unit 48. The focus lens
drive circuit 55 receives feedback signal from the rotary encoder
67, and stops the rotation of the motor 41 when the amount of
rotation of the motor 41 is reached the predetermined amount of
rotation corresponding to the focus signal from the CPU 52.
Therefore, the focusing is completed such that the moving of the
focus lens 66 is stopped at the position which corresponds one to
one with the focus signal. Obviously, the driving and stopping the
focus motor 41 may be also controlled by monitoring moving distance
of the focus lens 66.
In addition, the moving of the focus lens 66 may be controlled
manually by operating the focusing dial 23 manually and driving the
focus motor 41 according to the operating amount of the focusing
dial 23, with manually monitoring the focusing of the image
projected on the projection surface 9. When the zooming dial 22 is
operated manually, the zoom motor 42 is driven according to the
operating amount of the zooming dial 22 such that a variable power
lens which is part of the projection lens 21 is moving for zooming.
In case of zooming, moving distance of the focus lens 66 for
focusing is varied. However, the focusing of the projection lens 21
can be performed because position information of the variable power
lens is inputted into the CPU 52, and the CPU 52 outputs the focus
signal, which is adjusted to the position information of the
variable power lens, to the focus lens drive circuit 55.
The accurate distance signal must be inputted into the CPU 52 so
that the projection lens 21 is focused accurately. Therefore, there
is a need that the effective distance measurement light reflected
from the projection surface 9 enters into the light receiving
element 13. As described above, especially in case that the
whiteboard is used as the projection surface 9, the diffused light
from the light emitting lens 24 is strongly reflected at the
whiteboard, and the reflected diffused light as the noise light is
enters into the light receiving element 13 with the effective
distance measurement light. This causes erroneous distance
measurement. To prevent this problem, in the present invention, the
light emitting optical axis 24a is inclined at the inclination
angle .theta. with respect to the optical axis 21a of the
projection lens 21.
As shown in FIG. 5, when emitting the distance measurement light
which has the light emitting optical axis 24a inclined at the
inclination angle .theta. with respect to a virtual line
perpendicular to the projection surface 9, although the noise light
from the light emitting lens 24 is directly reflected at point P on
the projection surface 9 and enters to the light receiving lens 25,
an noise image 70 formed by the noise light regularly reflected
from the point P is away from a photoelectric surface of the light
receiving element 13, because the optical axis 25a of the light
receiving lens 25 and the light receiving element are also inclined
at the inclination angle .theta. as same as the light emitting lens
24.
Accordingly, the inclination angle .theta. for preventing erroneous
distance measurement can be calculated by calculating the condition
that the noise image 70 formed on an imaging surface of the light
receiving lens 25 from the noise light regularly (directly)
reflected from the point P is away from an effective photodetecting
region of the light receiving element 13.
Each one of symbols in FIG. 5 represents as follows.
R: diameter of the light emitting lens 24
w: size of the light receiving element 13
f: size of the noise image 70
g: distance between an light emitting side end of the light
receiving element 13 and the optical axis 25a of the light
receiving lens 25
h: distance between the center of the noise image 70 and the
optical axis 25a of the light receiving lens 25
t: distance between the center of the light receiving element 13
and the optical axis 25a of the light receiving lens 25
v: distance between the light receiving lens 25 and the light
receiving element 13 (.apprxeq.focal length of the light receiving
lens 25)
L: base length
Z: set distance to the projection surface 9 (the point P)
i: interval between the optical axis 25a of the light receiving
lens 25 and the point P
The size f of the noise image 70 on the imaging surface of the
light receiving lens 25 is expressed as
[f.apprxeq.R.times.(v/(z.times.2))] by using above symbols. In
addition, an angle a is calculated as [a=tan-1(h/v)], because
[g=(w/2)-t] and that the width h on the imaging surface, in which
the noise light entered to the light receiving lens 25 from the
point p at the angle a affects the distance measurement, is
[h=g+(f/2)]. The angle a is a limit angle such that the noise light
from the point P adversely affects the distance measurement.
When an interval between a perpendicular line from the point P to a
line passing through centers of the light emitting and receiving
lenses 24, 25 and the optical axis 25a of the light receiving lens
25 is represented as I, and an angle between the perpendicular line
and a line from the point P to the center of the light emitting
lens 24 is represented as b, the angle b is calculated as
[b=tan-1((L+i)/z)] because [i=tan(a).times.z]. Because an angle c
shown in FIG. 5 is calculated as [c=(b-a)/2], an angle .theta. of a
beam of noise light regularly (i.e., specularly) reflected from the
point P is calculated as [.theta.=c+a]. Accordingly, even if the
diffused light from the light emitting lens 24 is regularly
reflected at the projection surface 9 and enters to the light
receiving lens 25, it will not enter the light receiving element
while entering the lens and thus will not adversely affect the
distance measurement. Rather in the situation where the light
emitting optical axis 24a and the optical axis 25a of the light
receiving lens 25 are both inclined at the angle .theta. with
respect to the projection surface 9, the diffused light from the
light emitting lens 24 that is regularly reflected at the
projection surface 9 and that enters the light receiving lens will
be incident away from the light receiving element 13.
EXAMPLE
Now a concrete example of the above angle .theta. is shown. When
the diameter R of the light emitting lens 24 is 10 mm, the size w
of the light receiving element (PSD) 13 in the direction as same as
the base length is 1.2 mm, the base length L is 37.2 mm, the
distance t between the center of the light receiving element 13 and
the optical axis 25a of the light receiving lens 25 is 0.27 mm, the
distance v between the light receiving lens 25 and the light
receiving element 13 is 18.9 mm, and the distance z between the
projector 2 and the point P on the projection surface 9 is 1000 mm,
the size f of the noise image 70 in the direction as same as the
base length becomes 0.0945 mm. Also, it is calculated that the
distance g between the light emitting side end of the light
receiving element 13 and the optical axis 25a of the light
receiving lens 25 is 0.33 mm, the distance h between the center of
the noise image 70 and the optical axis 25a of the light receiving
lens 25 is 0.37725 mm, and the angle a is 1.14349.degree..
In addition, the angle b is 3.271485.degree. and the angle c is
1.063997.degree. because of distance i is 19.96032 mm. Accordingly,
the angle .theta. becomes 2.207488.degree.. Therefore, the
erroneous distance measurement is prevented such that the noise
image 70 is not formed on the photoelectric surface of the light
receiving element 13, in case if the light emitting optical axis
24a is inclined at the angle .theta., which is 2.207488.degree.,
with respect to the optical axis 21a of the projection lens 21. It
is only necessary to incline the distance measurement unit 39 at
least the angle .theta. with no concern about the direction of the
base length L, because above evaluation does not influenced by
directions to incline the light emitting optical axis 24a with
respect to the optical axis 21a of the projection lens 21.
For practical purposes, the angle .theta. is preferable to be wider
than 2.5.degree., and is more preferable to be wider than
4.degree., because there may be errors of relative positions of the
IRED 10 and the light emitting lens 24 and that of the light
receiving lens 25 and the light receiving element 13, and there may
be unsharpness of the distance measurement light and patterns of
the noise light on the projection surface 9. In addition, if the
angle .theta. become extremely wide, the distance measurement light
may become away from the region for projecting the image because
there become a large gap in distance between the distance
measurement light and the optical axis 21a of the projection lens
21. Therefore, the angle .theta. should be narrower than 20.degree.
for practical purposes.
To incline the light emitting optical axis 24a, an optical axis of
the light emitting lens 24 may be shifted with respect to the IRED
10 with that the optical axis of the light emitting lens 24 is
parallel to the optical axis of the projection lens 21, even though
that the distance measurement unit 39 is fixed to the support plate
43 such that the distance measurement unit 39 is inclined with
respect to the support plate 43 as described above is easier. In
general, the projector often be mounted on tables, but there is
also a case that the projector is embedded in a ceiling to project
the image downwardly. The present invention also can be applied to
the embedment-in-ceiling type projector.
In addition, the direction to arrange the light emitting lens 24
and the light receiving lens 25 (the direction of the base length
L) is not limited to longitudinal direction shown as the above
embodiment. As shown in FIG. 9, there may be a case that the light
emitting lens 24 and the light receiving lens 25 are arranged in
horizontal direction with respect to the mount surface, and the
optical axis of the light emitting lens 24 and the optical axis 25a
of the light receiving lens 25 become inclined perpendicularly
upward at the inclination angle .theta. with respect to the optical
axis 21a of the projection lens 21. In this case, as shown in FIG.
10, even if the noise light vertically entered to the projection
surface 9 is regularly reflected at the projection surface 9 and
enters to the light receiving lens 25, the noise light does not
adversely affect the distance measurement because the light
receiving element 13 is inclined upward same as the light receiving
lens 25.
* * * * *